1. Field of the Invention
The invention relates to a liquid crystal composition suitable for use in an active matrix (AM) device, and an AM device containing the composition. More specifically, the invention relates to a liquid crystal composition having a negative dielectric anisotropy, and also relates to a device of an in plane switching (IPS) mode, a vertical alignment (VA) mode or a polymer sustained alignment (PSA) mode containing the composition.
2. Related Art
In a liquid crystal display device, classification based on an operating mode of liquid crystals includes phase change (PC), twisted nematic (TN), super twisted nematic (STN), electrically controlled birefringence (ECB), optically compensated bend (OCB), in-plane switching (IPS), vertical alignment (VA), polymer sustained alignment (PSA) and so forth. Classification based on a driving mode of the device includes a passive matrix (PM) and an active matrix (AM). PM is further classified into static, multiplex and so forth, and AM is classified into a thin film transistor (TFT), a metal insulator metal (MIM) and so forth. TFT is further classified into amorphous silicon and polycrystal silicon. The latter is classified into a high temperature type and a low temperature type according to a production process. Classification based on a light source includes a reflection type utilizing a natural light, a transmission type utilizing a backlight and a semi-transmission type utilizing both the natural light and the backlight.
These devices contain a liquid crystal composition having suitable characteristics. The liquid crystal composition has a nematic phase. General characteristics of the composition should be improved to obtain an AM device having good general characteristics. Table 1 below summarizes a relationship between the general characteristics of the two. The general characteristics of the composition will be further explained based on a commercially available AM device. A temperature range of a nematic phase relates to the temperature range in which the device can be used. A desirable maximum temperature of the nematic phase is 70° C. or more and a desirable minimum temperature is −10° C. or less. The viscosity of the composition relates to the response time of the device. The rotation viscosity of the composition also relates to the response time of the device. A short response time is desirable for displaying a moving image. Accordingly, a small viscosity of the composition is desirable. A small viscosity at a low temperature is more desirable.
TABLE 1General Characteristics of Liquid Crystal Compositionand AM DeviceGeneral Characteristics ofGeneral Characteristics of anNoa CompositionAM Device1Temperature range of aUsable temperature range isnematic phase is widewide2Viscosity is small1)Response time is short3Optical anisotropy isContrast ratio is largesuitable4Dielectric anisotropy isThreshold voltage is low,positively or negativelyelectric power consumption islargesmall, and contrast ratio islarge5Specific resistance isVoltage holding ratio is large,largeand a contrast ratio is large6It is stable to ultravioletService life is longlight and heat1)A liquid crystal composition can be injected into a cell in a shorter period of time.
The optical anisotropy of the composition relates to the contrast ratio of the device. A product (Δn×d) of the optical anisotropy (Δn) of the composition and the cell gap (d) of the device is designed to maximize the contrast ratio. A suitable value of the product depends on the kind of operation mode. In a device having a VA mode, a suitable value is in a range of from 0.30 μm to 0.40 μm, and in a device having an IPS mode, a suitable value is in a range of from 0.20 μm to 0.30 μm. In this case, a composition having a large optical anisotropy is desirable for a device having a small cell gap. A large dielectric anisotropy of the composition contributes to a low threshold voltage, a small electric power consumption and a large contrast ratio of the device. Accordingly, a large dielectric anisotropy is desirable. A large specific resistance of the composition contributes to a large voltage holding ratio and a large contrast ratio of the device. Accordingly, a composition having a large specific resistance is desirable at room temperature and also at a high temperature in the initial stage. A composition having a large specific resistance is desirable at room temperature and also at a high temperature after it was used for a long time. A stability of the composition to an ultraviolet light and heat relates to a service life of the liquid crystal display device. When the stability is high, the device has a long service life. These characteristics are desirable for an AM device used in a liquid crystal projector, a liquid crystal television and so forth.
In an AM device having a TN mode, a composition having a positive dielectric anisotropy is used. In an AM device having a VA mode, a composition having a negative dielectric anisotropy is used. In an AM device having an IPS mode, a composition having a positive or negative dielectric anisotropy is used. In an AM device having a PSA mode, a composition having a positive or negative dielectric anisotropy is used. Examples of the liquid crystal composition having a negative dielectric anisotropy are disclosed in the following documents:    No. 1: JP 2001-262145 A; No. 2: JP 2001-115161 A.
A desirable AM device is characterized as having a wide usable temperature range, a short response time, a high contrast ratio, a low threshold voltage, a large voltage holding ratio, a long service life, and so forth. A shorter response time is desirable even by one millisecond. Thus, a composition having characteristics such as a high maximum temperature of a nematic phase, a low minimum temperature of a nematic phase, a low viscosity, a large optical anisotropy, a proper dielectric anisotropy, a large specific resistance, a high stability to ultraviolet light, a high stability to heat, and so forth is especially desirable.
One of the object of the invention is to provide a liquid crystal composition that satisfies at least one of the characteristics such as a high maximum temperature of a nematic phase, a low minimum temperature of a nematic phase, a small viscosity, a proper optical anisotropy, a large dielectric anisotropy, a large specific resistance, a high stability to ultraviolet light and a high stability to heat and so forth. Another object of the invention is to provide a liquid crystal composition that is properly balanced regarding at least two of the characteristics. Another advantage of the invention is to provide a liquid crystal display device that contains the liquid crystal composition. Another of the object of the invention is to provide a liquid crystal composition that has a proper (a small or a large) optical anisotropy, a large dielectric anisotropy, a high stability to ultraviolet light and so forth, and is to provide an AM device that has a short response time, a large voltage holding ratio, a large contrast ratio, a long service life and so forth.